Analog Circuits Multiple Choice Questions on “Drift and Diffusion Current”.
1. Drift current is due to ___________
A. Applied electric field over a given distance
B. Random motion of electrons
C. Random motion of holes
D. Recombination of holes and electrons
Answer: A
Clarification: Drift current is a type of electric current due to the movement of charge carriers which occurs because of applied electric field across the p-n junction often stated as electromotive force over a given distance.
2. Diffusion current is due to ___________
A. Applied electric field over a given distance
B. Variation in carrier concentration
C. Random motion of holes
D. Recombination of holes and electrons
Answer: B
Clarification: Diffusion current is due to the actual movement of carrier charges from one side to another. The direction of diffusion depends on the slope of the carrier concentration that is the gradient of density of carriers.
3. What makes up the total current in a semi-conductor?
A. Only diffusion current
B. Only drift current
C. Drift+diffusion current
D. Drift+diffusion+biasing current
Answer: C
Clarification: In an unbiased semi-conductor the drift current is balanced by diffusion current and hence there is no current flowing in the semi-conductor in this condition, but in biased condition both these currents are unbalanced and hence the total current flowing is the vector sum of both drift and diffusion current.
4. Conductors also have drift current.
A. True
B. False
Answer: A
Clarification: In good conductors there are many electrons moving freely in the conduction band and thus on application of an electric field these free electrons move whereas in semi-conductors drift current flows because of less number of free electrons.
5. The equation Jn=qnµnE (A/cm2) represents___________
A. Drift current
B. Drift current density
C. Diffusion current
D. Diffusion current density
Answer: b
Clarification: Here ‘q’ is the charge on carrier, ‘n’ is the number of carriers, ‘µn’ is the mobility constant and ‘E’ is the electric field intensity. These are the factors which comprise the drift current and hence the equation represents the drift current density.
6. Is the statement “Diffusion current produces Drift current” true?
A. Yes
B. No
C. Cannot Say
D. Insufficient Data
Answer: A
Clarification: The movement of charge carriers is diffusion current and this movement produces an electric field which is the root cause of drift current. This is also proved by the fact that both drift and diffusion current are equal and opposite in an unbiased semi-conductor.
7. What is the average net velocity in the direction of the electric field?
A. Velocity of electrons
B. Velocity of holes
C. Drift velocity
D. Collision velocity
Answer: C
Clarification: The carriers accelerate in the direction of electric field between collisions but for each time interval τc (collision time) there is a collision which randomises the velocity of the carrier. This average net velocity in the direction of electric field is Drift velocity.
8. What is mobility?
A. Ease of carrier drift
B. Ease of current flow
C. Ease of access to the junction
D. Ease of movement
Answer: A
Clarification: Mobility is the ease with which carriers can drift. The higher the collision time, the greater is the mobility also the lighter is the carrier, the greater is its mobility. Thus on the application of an electric field it’s easier for carriers to drift.
9. Why does a gradient occur in a semi-conductor?
A. Because of current flow
B. Because of diffusion current
C. Because of drift current
D. Because of difference in concentrations
Answer: D
Clarification: As there is a different level of doping in the p and n regions of a semi-conductor, the carriers (either holes or electrons) move from a region of high concentration to a region of low concentration giving rise to diffusion current.
10. How does diffusion current produce the depletion region?
A. The diffusion causes the holes and electrons to collect at the junction
B. The diffusion is because of the depletion region
C. The depletion region aids diffusion
D. The statement is not true
Answer: A
Clarification: The diffusion of carriers from one side to another makes the holes and electrons to collect on either side of the junction creating the depletion region. This is further widened or shortened depending on the biasing.